Given a set of subgrades, you have to be able to find the overall grade without ambiguities.

I propose a method of determining the overall grade from the subgrades. You guys can check if it works from known examples. This rule doesn't punish corner harder than others.

We know the four subgrades and the overall grade must range between 1-10 with 0.5 increments.

Method:

You rank the four numbers from the subgrades in a decreasing order. The third subgrade will be the third number in that order, and the fourth subgrade will be the fourth number.

Rule 1: The overall grade must be greater than or equal to the fourth subgrade and it can be at most 1.0 higher than the fourth subgrade.

Rule 2: The overall subgrade must be less than or equal to the third subgrade; but it cannot be equal to the third subgrade if the fourth subgrade is 1.0 below or more.

Conclusion: This narrows down to at most 3 numbers as candidates for the overall subgrade in different scenarios. (3-number scenario happens when the third and fourth subgrades are 1.5 apart or more.) The overall grade will be the highest number.